July 2022 was the driest July in England since 1935. Combined with record high temperatures, we are hearing talk of a drought comparable to the Great Drought of 1976, with fears of disruption to public water supplies and poor yields, particularly for fruit and vegetables. . But not all droughts are the same and not all farmers are affected by the same type of drought.
To a meteorologist, drought is usually defined as a period of significantly below average rainfall. But low rainfall even over an entire season does not necessarily mean that water supplies will be low, or that industry or agriculture will suffer, as there may be a lot of water already stored in reservoirs and groundwater.
Of course, such reserves are of little help to grasslands, cereals and other crops which are completely rain-fed and which are badly affected when we get a dry spring and summer. The past 12 months have been particularly dry across much of the UK, and since May 2021 only October and February have recorded above average rainfall.
Things are even worse when combined with the high temperatures and abundant sunshine we’ve seen this year, which increases evaporation and depletes the soil of the water required for plant growth – a so-called “agricultural drought”.
We have evaluated the combined effect of low rainfall and warm, sunny weather using potential soil moisture deficit (PSMD), which is a cumulative measure (in millimeters) of the balance between rainfall input to the soil and potential losses through evaporation and plant transpiration. .
When evaporation exceeds precipitation, the soil becomes drier and PSMD increases. When it rains, it decreases. Generally, PSMD begins to increase from late March or early April, and peaks in August or September when the soil is at its driest. A high PSMD means that rainfed crops such as cereals and grasses, as well as our gardens, will suffer.
Using data from weather stations in Cambridge, we estimate that PSMD in 2022 (so far) has behaved very similarly to 1976. The deficit began to increase in early March and has continued to grow until the end of July.
This is in contrast to the last drought in 2018, when the spring was wetter and soil drying was delayed. PSMD currently stands at about 350mm, which is around 50% higher than the average peak between 1981 and 2010. So for farmers who rely solely on rainfall, 2022 looks like it could be as severe an agricultural drought as 1976.
Irrigated agriculture may be restricted
Most grassland and “breadcre” crops such as cereals and oilseeds are grown in the UK without irrigation. It’s not that they don’t need the water, but that investing in irrigation equipment is financially unattractive.
But to ensure yields and especially crop quality, much of Britain’s potato, vegetable and fruit crops receive extra water from irrigation during dry periods. Dry soil also means that the demand for water for irrigated crops will be higher, competing with reduced available water resources for other sectors.
To the water resource manager, a “hydrological drought” is when the water available in rivers, reservoirs and groundwater is insufficient to meet demand – including the demand to maintain a healthy aquatic ecosystem.
Potatoes account for more than half of the UK’s irrigated area and volume of irrigation water used. In a dry year, we estimate that one hectare of potatoes (just over half a football field) needs more than 2 million liters of irrigation water to maintain yield and quality. That’s more than 40 liters for every kg of potatoes.
As UK irrigated agriculture and horticulture need a lot of water, but are considered a non-essential user, irrigated farmers risk mandatory restrictions during a drought, with potentially severe financial consequences.
Here we see a difference between 1976 – which followed a very dry 1975 – and 2022. The Met Office described rainfall in 2021 as “unremarkable”. This, together with better water metering and investment in infrastructure to move water from areas of availability to demand, means that water resources are in better condition now than they were in 1976.
The maps below show the status of river flows across the country in February 1976 and February 2022. Pink to red indicates river flows that were below normal (pink) to exceptionally low (red) for the time of year.
So while this year’s dry and hot weather has been similar to 1976 with similar effects on our gardens and farming, last winter ended with water resources that were mostly around normal for the time of year. This means that we do not expect extensive mandatory restrictions on irrigated farms, although some restrictions may be imposed to protect supplies in certain catchments.
Despite the water resource situation not being as dire in 2022 as it was in 1976, demand across all uses must be managed to prevent a severe hydrological drought this year. It is also wise to manage our water resources carefully in the summer of 2022, not only to avoid restrictions this year, but also to reduce the risk of tighter restrictions next year if the UK follows this dry summer with a dry winter.
This article is republished from The Conversation under a Creative Commons license. Read the original article.
Tim Hess has received funding from the Natural Environment Research Council, the Biotechnology and Biological Sciences Research Council and the Economic and Social Research Council. He is associated with Water and Sanitation for the Urban Poor.
Ian Holman has previously received funding from the Natural Environment Research Council